Charge pump is also referred to as switching-capacitor voltage converter, which is a DC-DC voltage converter based on capacitor energy storage. A charge pump generally controls the charging and discharging of a capacitor in switch mode so as to increase or decrease an input voltage of the charge pump with a certain factor, thereby obtaining a desired output voltage. Because of its high efficiency, small size, low cost and small EMI, charge pump is widely used in large-current and low-voltage portable products.
In U.S. Pat. No. 7,746,041 there is disclosed a charge-pump voltage divider topology which can be used in large-current and low-voltage applications. FIG. 1 schematically shows this charge-pump voltage divider 100, comprising switches M1-M4, a capacitor C1, and a capacitor Cout. The switches M1-M4 are sequentially connected in series between an input terminal and ground GND, wherein the input terminal is configured to receive an input voltage Vin. One terminal of the capacitor C1 is coupled to a common connection of the switches M1 and M2, and the other terminal of the capacitor C1 is coupled to a common connection of the switches M3 and M4. The capacitor Cout is paralleled to a load 102, wherein one terminal of capacitor Cout is coupled to a common connection of the switches M2 and M3, and the other terminal of capacitor Cout is connected to ground GND. A first control signal Q is used to control the ON state and OFF state of the switches M1 and M3, and a second control signal Q is used to control the ON state and OFF state of the switches M2 and M4, wherein the first control signal Q and the second control signal Q are complementary, i.e. when the first control signal Q is logic high, the second control signal Q is logic low, and vice versa. During operation, when the first control signal Q is logic high, the switches M1 and M3 are turned on, inversely, the switches M2 and M4 are turned off by the second control signal Q. Thus, the capacitors C1 and Cout are connected in series, and the input voltage Vin charges the capacitors C1 and Cout. When the first control signal Q is logic low, the switches M1 and M3 are turned off, inversely, the switches M2 and M4 are turned on by the second control signal Q. Thus, capacitors C1 and Cout are connected in parallel, the one with higher voltage between the capacitors C1 and Cout discharges to the one with lower voltage. The above operation process repeats until the voltage across the capacitor C1 equals to the voltage across capacitor Cout, and equals
      1    2    ⁢      Vin    .  
However, when the charge-pump voltage divider 100 starts up, the switches M1 and M3 are turned on instantaneously once the first control signal Q is logic high, yet the voltage across capacitor Cout is still zero. In this situation, an instantaneous large inrush current can occur, which may burn the switches M1 and M3 down.
Accordingly, a circuit and a method for controlling the start-up of the charge-pump voltage divider are desired.